a) Field of the Invention
The present invention relates to a liquid immersion objective lens system having a low magnification, for example, a liquid immersion objective lens system for microscopes. Furthermore, the present invention relates also to an optical apparatus using the liquid immersion objective lens system having the low magnification, for example, a form observing apparatus such as a microscope or a measuring apparatus such as a site meter.
b) Description of the Prior Art
Objective lens system having high magnifications of 40xc3x97 to 100xc3x97 are used for observing chromosomes and genes in cells which are minute objects. Measuring apparatuses such as a site meter may use objective lens systems which have low magnifications around 20xc3x97. Furthermore, objective lens systems of this kind are mostly liquid immersion objective lens systems since these lens systems must have large NAs to maintain required brightness and resolution. Furthermore, objective lens systems having low magnifications are frequently used to search for the above described objects to be observed.
When a minute object is observed with a liquid immersion objective lens system and then another object to be observed is to be searched again with an objective lens system having a low magnification, it is necessary to wipe off a liquid from a specimen each time, whereby a working efficiency is lowered remarkably. When the liquid is not wiped off, there occurs a defect such as degradation of an image of an observed object or adhesion of the liquid to a tip of the objective lens system having the low magnification, thereby resulting in lowering of a quality of an apparatus. Lowering of the working efficiency or the quality is not preferable for an automatic inspection apparatus in particular.
An object of the present invention is to provide a liquid immersion objective lens system which uses an immersion liquid having a refractive index ND for the d-line (587.6 nm) of a value within a range defined by a condition (1) shown below, and satisfies the following conditions (2) and (3):
1.34xe2x89xa6NDxe2x89xa61.80xe2x80x83xe2x80x83(1) 
xcex2xe2x89xa615xe2x80x83xe2x80x83(2) 
WDLxe2x89xa60.35 mmxe2x80x83xe2x80x83(3) 
Another object of the present invention is to provide an optical apparatus which comprises a plurality of objective lens systems and observing means, wherein the objective lens systems include a liquid immersion objective lens system having a magnification not higher than 15xc3x97 and a liquid immersion objective lens system having a magnification not lower than 15xc3x97, and an immersion liquid has a refractive index ND for the d-line satisfying the following condition (1):
1.34xe2x89xa6NDxc3x971.80xe2x80x83xe2x80x83(1) 